Check for the existence of `$post` before using it in `get_the_ID()`. Return `false` if it doesn't exist.

Props UmeshSingla.
Fixes #17034.

Built from https://develop.svn.wordpress.org/trunk@28844


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In `kses.php`, ensure that `$allowedposttags`, `$allowedtags`, and `$allowedentitynames` are added to the global namespace.

Props Jaza613.
Fixes #28582.


Built from https://develop.svn.wordpress.org/trunk@28845


git-svn-id: http://core.svn.wordpress.org/trunk@28649 1a063a9b-81f0-0310-95a4-ce76da25c4cd
When `wp_oembed_add_provider()` or `wp_oembed_remove_provider()` is called before the `plugins_loaded` hook has, store the values statically on the `WP_oEmbed` object and add them just-in-time when the object is instantiated. 

This ensures that all plugins have an accurate provider list when `apply_filters( 'oembed_providers', $providers )` is called. 

Props kovshenin.
Fixes #28284.

Built from https://develop.svn.wordpress.org/trunk@28846


git-svn-id: http://core.svn.wordpress.org/trunk@28650 1a063a9b-81f0-0310-95a4-ce76da25c4cd